This application is based on application No. H11-366288 filed in Japan on Dec. 24, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a decentered optical system including a decentered lens element, and particularly to a focusing method that keeps the image point substantially in a fixed position even when the object distance varies.
2. Description of the Prior Art
Conventionally, as focusing methods for centered optical systems, a plurality of methods have been proposed such as those by which a whole lens system or a front lens unit is moved out. However, such focusing methods for centered optical systems by which a lens system or lens unit is moved along the optical axis are not applicable to optical systems, such as decentered optical systems, that have no optical axis or an optical axis that can not be defined as a straight line.
Therefore, for decentered optical systems, a focusing method has been proposed by which part of a decentered optical system is designed as a centered optical system and part of the constituent lens elements of this centered optical system are moved along the optical axis thereof.
However, the above-mentioned focusing method proposed for decentered optical systems requires incorporation of a centered optical system in a decentered optical system, and therefore makes the entire optical system larger and in addition restricts the freedom of design of the decentered optical system. Furthermore, in such a centered optical system portion, the central ray needs to be aligned with the optical axis, which makes the entire optical system still larger.
An object of the present invention is to provide a focusing method for a decentered optical system that permits the decentered optical system to be designed without incorporating a centered optical system therein and without sacrificing the freedom of design thereof.
To achieve the above object, according to one aspect of the present invention, a focusing method for a decentered optical system including a decentered lens element is provided with: translationally moving and inclining at least one lens element for different object distances in such a way that variation in the position of the primary image point is kept within the range defined by the following condition:
|xcex94fbxc3x97sin("THgr"max)|maxxe2x89xa60.8 
where xcex94fb represents the maximum distance traveled by the primary image point during focusing; and "THgr"max represents the maximum angle of peripheral rays traveling from the center of an object to the center of the image plane relative to a base ray.
The primary image point is defined by the following calculation method. Relative to the ray (base ray) that travels from the center of an object through the center of an aperture stop to the image plane, the values attributable to the first surface of the optical system and then the values attributable to the second and the following surfaces are determined. Then, the back focal length of the entire system, i.e. the composite back focal length of those of the first to the last (the n-th) surfaces, and the magnification of the entire system at that time are defined. If the object is located at infinity, the focal length is used instead of the magnification.
According to another aspect of the present invention, a focusing method for a decentered optical system including a decentered lens element is provided with: translationally moving and inclining at least one lens element for different object distances in such a way that the primary image point is kept substantially in a fixed position, wherein the following condition is fulfilled:
|xcex94fs/xcex94dl|xe2x89xa620 
where xcex94dl represents the variation in the object distance; and xcex94fs represents the amount of movement, during focusing, of at least one point within the effective area on the surface of the lens element that is moved for focusing.
According to another aspect of the present invention, a decentered optical system is provided with: a decentered lens element, wherein at least one lens element is translationally moved and inclined for different object distances in such a way that variation in a position of a primary image point is kept within a range defined by the following condition:
|xcex94fbxc3x97sin("THgr"max)|maxxe2x89xa60.8 
where xcex94fb represents a maximum distance traveled by the primary image point during focusing; and "THgr"max represents a maximum angle of peripheral rays traveling from a center of an object to a center of an image plane relative to a base ray.
According to another aspect of the present invention, a decentered optical system is provided with: a decentered lens element, wherein at least one lens element is translationally moved and inclined for different object distances in such a way that a primary image point is kept substantially in a fixed position, wherein the following condition is fulfilled:
|xcex94fs/xcex94dl|xe2x89xa620 
where xcex94dl represents a variation in the object distance; and xcex94fs represents an amount of movement, during focusing, of at least one point within an effective area on a surface of the lens element that is moved for focusing.